This invention relates to a valve seat arrangement for a reciprocating machine and more particularly to an improved valve seat for an internal combustion engine.
In internal combustion engines it frequently is the practice to employ aluminum or aluminum alloys as the material for a number of the major engine castings such as the cylinder heads. When the cylinder heads are formed from aluminum or aluminum alloys, however, certain components of the cylinder head are formed from a dissimilar material so as to improve performance. For example, the valve seats of the cylinder head are normally formed from a harder, less heat conductive material such as iron or ferrous iron alloys. By utilizing such harder materials, the valve seat life can be extended. However, the attachment of the dissimilar valve seat insert into the cylinder head presents a number of problems.
Conventionally, it has been the practice to form the cylinder head passages with recesses adjacent the seating area into which the insert rings which form the valve seat are press fit. The use of press fitting has a number of disadvantages. First, it requires relatively large valve seat inserts in order to withstand the pressing pressures. In addition, the press fit mutt be such that the insert ring will not fall out when the engine is running. As a result there are quite high stresses exerted both on the cylinder head and on the insert ring The stresses can result in loads which may eventually cause cracks in the cylinder head.
These types of construction also limit the maximum size and spacing of the valve seats in order to ensure adequate cylinder head material between adjacent valve seats to reduce the likelihood of cracking. In addition, the large seats compromise the configuration of the intake passages, Particularly at the critical valve seating area. Finally, these constructions result in somewhat poor heat transfer from the valve to the cylinder head due to the poor thermal conductivity of the valve seat material.
In addition, the interface between the insert ring and the cylinder head frequently leaves voids or air gaps which further reduce the heat transfer and thus cause the valves to run at a higher temperature. This higher temperature operation of the valves requires the valves to be made heavier and stronger and thus reduce the performance of the engine and increase its size and costs.
It has been proposed, therefore, to utilize a technology wherein the insert ring is actually welded into the cylinder head. Such welding generally ensures against the likelihood of stresses which may cause cracking. Nevertheless, the welding technique itself requires rather large inserts and thus a number of the disadvantages with pressed in inserts also are found with welded inserts. Furthermore, the heat transfer problems are also prevalent and in some instances can become worsened.
With a welding technique, there is actually formed a metallurgical alloy between the material of the insert ring and the cylinder head. Because of the fusion process, air pockets or voids may occur and heat transfer is reduced. In addition, the alloy at the interface between the insert ring and the cylinder head also has poor thermal conductivity and thus a number of the problems present with pressed in inserts are also present with welded inserts.
Another technique has been suggested and employed wherein a laser cladding technique is employed. Again, however, this requires actual melting of the materials and as a result many of the problems with other more conventional techniques are also present.
It has been proposed, therefore, to employ a technique wherein the insert ring is metallurgically bonded but not alloyed to the cylinder head material. This is accomplished by pressing the insert into place and passing an electrical current through the insert which is sufficient to cause the cylinder head material to plastically deform upon insertion of the insert ring. The plastically deformed phase of the cylinder head material forms a metallurgical bond at the interface with the insert ring without any significant resulting alloying of the cylinder head material to that of the insert ring. Such an arrangement is disclosed in our co-pending application entitled, "Valve Seat Bonded Cylinder Head aid Method for Producing Same," Application No., 08/483,246, filed Jun. 7, 1995 and assigned to the assignee hereof. In addition, certain of these techniques are also described in co-pending application entitled "VALVE SEAT," Application Ser. No. 08/278,026, filed Jul. 20, 1994, in the names of Shuhei Adachi & Junichi Inami and also assigned to the Assignee hereof.
These techniques have a number of advantages over the conventional structures. First, they permit the use of much smaller insert rings since the pressing pressure is reduced and thus the shape of the intake passage, particularly the shape of the cylinder head passages, particularly in the critical area of the valve seats are not compromised. In addition, the bond strength is considerably higher than more conventional methods. Furthermore, this technique, because of the improved way in which the adhesion is formed, permits the use of much smaller insert rings and thus permits the valve seat openings to be positioned closer to each other without the likelihood of causing defects in the cylinder head which may manifest themselves during the engine running and life.
In the aforedescribed co-pending applications, two different types of pressing techniques are employed for pressing and bonding the insert ring into position. In accordance with one of these techniques, called the "end pressing" method, the pressure is applied generally to the insert ring along the end face of the ring and about a pressing surface that extends generally perpendicularly to the flow axis of the insert ring. With this type of arrangement, the outer periphery of the insert ring is formed with a first section that extends from the pressing surface generally axially and which is inclined at a relatively shallow acute angle to the flow axis direction. This surface terminates in a generally radially extending surface which preferably is incline at a small acute angle to the surface to which the pressing force is applied.
With this type of pressing method, the insert ring generally experiences primarily only a compressive force and hence it is not stressed in a manner where the ring is likely to be damaged. However, the bonding strength along the periphery of the insert ring is not completely uniform. The bonding strength is found to be substantially higher along the bottom surface then along the incline side surface.
The other type of pressing technique which is employed is a "tapered pressing" technique wherein the pressing tool has a tapered surface which engages a tapered surface of the insert ring which tapered surface forms a part of the final flow passage. This technique permits the use of an exterior surface for the insert ring wherein the lower surface portion can be inclined at a greater angle to the perpendicular plane passing through the flow axis and a mole uniform bond results. However, the loads placed on the insert are tensile rather than compressive and hence with conventional materials, the stressing might cause problems with the insert ring.
It is, therefore, a principal object of this invention to provide and improve insert ring material that has properties that will ensure against damage from over stressing with either type of pressing method.
It is a further object of this invention to provide and improve insert ring material for a bonded valve seat wherein the insert ring is formed with a material that has a substantially greater elongation at rupture than conventional materials and thus, produces the aforenoted results.
It is yet a further object of this invention to provide an improved method for forming engine valve seats.